The overall goal of this 2-year population based epidemiologic study is to lay the foundation for future cross-sectional and prospective research on the associations between blood pressure and high blood pressure and diurnal variation in the urinary excretion of sodium, potassium, and water. Specific goals are first, to determine if hypertensives and normotensives of similar ages have, on average, different ratios of 24-hour to overnight excretion of sodium, potassium, and water; second, to determine the number of 24-hour collections needed to characterize individuals with respect to the ratios of 24-hour to overnight excretion; and third, to determine if the ratios of 24-hour to overnight excretion vary by age and other factors related to blood pressure and high blood pressure. Two hundred fifty eight white males ages 30-59, randomly selected from the labor force of the Chicago Peoples Gas Company, are to be the participants of this study. One half of the participants are to be age 30-44 and the other half age 45-59. In addition, one third of the participants in each age group are to be in each of the three groups: I. DBP less than 80 mm Hg and no anti-hypertensive medication; II. DBP 80- 89 mm Hg and no anti-hypertensive medication; and III. DBP greater than or equal to 90 mm Hg or on anti-hypertensive medication. Each participant is to provide three 24-hour urine collections, divided into daytime and overnight specimens, for the assessment of daytime and overnight excretion of sodium, potassium, creatinine, and water; to have four blood pressure measurements; to have height and weight measured; and to complete a questionnaire dealing with demographic variables, alcohol intake, history and treatment of high blood pressure, use of medications, family history of high blood pressure, cigarette use, and changes in dietary habits. Three to four participants are to be recruited and examined each week over an 18-month period beginning in the fourth month. All participant examinations are to be on Mondays with collection of overnight urines on Monday, Tuesday, and Wednesday, and collection of daytime urines on Tuesday, Wednesday, and Thursday. The primary method of data analysis is to be analysis of variance.